From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.6 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,TVD_SUBJ_WIPE_DEBT,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D054CC433E0 for ; Sat, 27 Feb 2021 10:32:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9A15464EAF for ; Sat, 27 Feb 2021 10:32:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230001AbhB0KcB (ORCPT ); Sat, 27 Feb 2021 05:32:01 -0500 Received: from pegase1.c-s.fr ([93.17.236.30]:43071 "EHLO pegase1.c-s.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229795AbhB0Kb7 (ORCPT ); Sat, 27 Feb 2021 05:31:59 -0500 Received: from localhost (mailhub1-int [192.168.12.234]) by localhost (Postfix) with ESMTP id 4DnjVW038rz9ty5v; Sat, 27 Feb 2021 11:31:15 +0100 (CET) X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id 4h_HczVVdAdx; Sat, 27 Feb 2021 11:31:14 +0100 (CET) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 4DnjVV54hBz9ty5t; Sat, 27 Feb 2021 11:31:14 +0100 (CET) Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id EC11A8B77A; Sat, 27 Feb 2021 11:31:15 +0100 (CET) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id W-ItOljZNSxg; Sat, 27 Feb 2021 11:31:15 +0100 (CET) Received: from [192.168.4.90] (unknown [192.168.4.90]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 186948B75B; Sat, 27 Feb 2021 11:31:15 +0100 (CET) Subject: Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON() To: Segher Boessenkool Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , npiggin@gmail.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org References: <694c7195c81d1bcc781b3c14f452886683d6c524.1613029237.git.christophe.leroy@csgroup.eu> <20210211114910.GA28121@gate.crashing.org> <6126ca14-419a-9e15-7ffa-b295f26a552e@csgroup.eu> <20210211143059.GE28121@gate.crashing.org> From: Christophe Leroy Message-ID: Date: Sat, 27 Feb 2021 11:31:08 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <20210211143059.GE28121@gate.crashing.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le 11/02/2021 à 15:30, Segher Boessenkool a écrit : > On Thu, Feb 11, 2021 at 03:09:43PM +0100, Christophe Leroy wrote: >> Le 11/02/2021 à 12:49, Segher Boessenkool a écrit : >>> On Thu, Feb 11, 2021 at 07:41:52AM +0000, Christophe Leroy wrote: >>>> powerpc BUG_ON() is based on using twnei or tdnei instruction, >>>> which obliges gcc to format the condition into a 0 or 1 value >>>> in a register. >>> >>> Huh? Why is that? >>> >>> Will it work better if this used __builtin_trap? Or does the kernel only >>> detect very specific forms of trap instructions? >> >> We already made a try with __builtin_trap() 1,5 year ago, see >> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20510ce03cc9463f1c9e743c1d93b939de501b53.1566219503.git.christophe.leroy@c-s.fr/ >> >> The main problems encountered are: >> - It is only possible to use it for BUG_ON, not for WARN_ON because GCC >> considers it as noreturn. Is there any workaround ? > > A trap is noreturn by definition: > > -- Built-in Function: void __builtin_trap (void) > This function causes the program to exit abnormally. > >> - The kernel (With CONFIG_DEBUG_BUGVERBOSE) needs to be able to identify >> the source file and line corresponding to the trap. How can that be done >> with __builtin_trap() ? > > The DWARF debug info should be sufficient. Perhaps you can post-process > some way? > > You can create a trap that falls through yourself (by having a trap-on > condition with a condition that is always true, but make the compiler > not see that). This isn't efficient though. > > Could you file a feature request (in bugzilla)? It is probably useful > for generic code as well, but we could implement this for powerpc only > if needed. > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99299 Christophe